The invention relates to the field of integrated optics and, in particular, to a multimode interference coupler having a tunable power splitting ratio and a method of tuning the ratio of the coupler.
Mode couplers are used extensively in integrated optics to split and combine light signals propagating through an optical waveguide structure. Different types of couplers are currently availablexe2x80x94e.g., symmetrical Y branches, directional couplers, star couplers and multimode interference couplers. Multimode interference (MMI) couplers have advantages such as compactness, large optical bandwidth and large design tolerance. Once fabricated, however, a conventional MMI coupler offers only a fixed power splitting ratio. Modification of the MMI coupler""s geometry is usually necessary in order to obtain different splitting ratios.
A mode coupler is often used in Mach-Zehnder electro-optic switches. When on/off ratios above 20 dB are desired in these electro-optic switches, it is desirable to have a mode coupler having a xe2x80x9ctunablexe2x80x9d power splitting ratioxe2x80x94i.e., variable splitting ratio without the need for physical modification of the mode coupler. In all-optical Mach-Zehnder type wavelength converters, a strong optical control pulse induces refractive index change in one arm of a Mach-Zehnder interferometer and allows switching of a data signal from one output arm to the other. However, the strong optical signal used to induce the phase-shift also modulates the intensity of the data signal to be controlled, resulting in a reduced extinction ratio. Asymmetric splitting ratios in the couplers are often required to attain ideal extinction ratios in order to compensate for the gain asymmetries from the carrier depletion in the semiconductor optical amplifiers.
The invention relates to a multimode interference coupler having a tunable power splitting ratio and a method of tuning the splitting ratio. The splitting ratio is tuned by varying an effective refractive index of at least a portion of the multimode interference coupler. The effective refractive index may be varied at some pre-determined locations of the coupler by supplying, for example, an electrical current, voltage or light to the specific locations of the coupler, resulting in tunability of the power splitting ratio of the coupler.